1. Field of the Invention
The present invention relates generally to a trench-type DRAM memory cell and its manufacturing method and, more particularly, to a vertical transistor DRAM structure and its manufacturing methods.
2. Description of Related Art
A dynamic random-access-memory (DRAM) cell including an access transistor -and a storage capacitor has become the most important storage element in electronic system, especially in computer and communication system. The DRAM density is increased very rapidly in order to decrease the cost per bit and, therefore, an advanced photolithography is needed to decrease the minimum-feature-size (F) of a cell.
The output voltage of a DRAM cell is proportional to the capacitance value of the storage capacitor of the DRAM cell and, therefore, the storage capacitor must have a satisfactory capacitance value to have stable operation of the cell as the applied voltage is scaled. Basically, the storage capacitor can be implemented in a trench-type or a stack-type. The trench-type is formed by forming a deep trench in a semiconductor substrate without increasing the surface area of the semiconductor-substrate surface. The stack-type is formed by implementing a capacitor structure over the access transistor and its nearby dummy-transistor structure through the conductive contact-plug over the node diffusion region of the access transistor. Basically, the cell size of the stack-type DRAM is limited by a dummy transistor being formed over the isolation region. Accordingly, the limit cell size of the stack-type DRAM is 8F2 for shallow-trench-isolation. However, the cell size of a trench-type lateral transistor DRAM is limited by the space between nearby deep-trench capacitors and the separation between the lateral access transistor and the deep-trench capacitor. Therefore, the limit cell size of a trench-type lateral transistor DRAM is also 8F2 if the separation between the lateral access transistor and the trench capacitor can""t be minimized.
A typical example of a trench-type lateral transistor DRAM cell is shown in FIG. 1, in which a deep trench is formed in a semiconductor substrate 100. A trench capacitor is formed in a lower portion of the deep trench, in which a lower capacitor node 101 is formed by a heavily-doped n+ diffusion region using an arsenic-silicate-glass (ASG) film as a dopant diffusion source; an upper capacitor node 103a is made of doped polycrystalline-silicon; and a capacitor-dielectric layer 102 is formed by a composite dielectric layer such as an oxide-nitride-oxide structure or a nitride-oxide structure. An oxide collar 104 is used to separate the lower capacitor node 101 from a source diffusion region 105a, 105b, and a capacitor-node connector 103b being made of doped polycrystalline-silicon is used to electrically connect the upper capacitor node 103a to a source conductive node 103c. The source conductive node 103c is made of heavily-doped polycrystalline-silicon to act as a dopant diffusion source for forming an n+source diffusion region 105a. A shallow-trench-isolation (STI) region 106 is filled with a CVD-oxide layer in order to separate nearby trench capacitors. Two gate-stacks 108, 109 are formed over an upper surface, in which one gate-stack 108 is acted as a passing word line and another gate-stack 109 being acted as an excess transistor. A common-source diffusion region 105b and a common-drain diffusion region 107 for a bit-line node are formed in an upper surface of the semiconductor substrate 100. From FIG. 1, it is clearly seen that the limit cell size is 8F2 if the space between two nearby trench capacitors is defined to be a minimum-feature-size (F) of technology used. It is clearly seen that the cell size can be further reduced if the separation between two adjacent deep trenches and the common-source region can be reduced.
Apparently, the common-source diffusion region 105b, 105a and the gate-stack 109 shown in FIG. 1 can be removed and are formed in the deep-trench region to become a vertical transistor DRAM structure, then the semiconductor surface area can be saved at least 2F2. However, a depth of the deep trenches becomes deeper, resulting in a further problem for forming a deeper trench. Moreover, the threshold-voltage and the punch-through voltage of the vertical transistor are difficult to be controlled, and a longer channel length is therefore used by the prior art. As a consequence, a deeper trench depth is required, and a slower read/write speed of a memory cell due to a longer channel length becomes another serious problem for the prior art.
It is, therefore, a major objective of the present invention to offer a vertical transistor DRAM structure for obtaining a cell size of 4F2.
It is another objective of the present invention to easily offer different implant regions for forming punch-through stops and adjusting threshold-voltages of the vertical transistor and the parasitic collar-oxide transistor in a self-aligned manner so a deeper trench is not required.
It is a further objective of the present invention to offer a manufacturing method for forming the vertical transistor DRAM structure with less masking photoresist steps.
It is yet another objective of the present invention to offer two different contactless vertical transistor DRAM array structures for high-speed read and write operations.
A vertical transistor DRAM structure and its contactless vertical transistor DRAM arrays are disclosed by the present invention. The vertical transistor DRAM structure comprises a trench structure and a common-drain structure, in which the trench structure includes a deep-trench region having a vertical transistor and a second-type shallow-trench-isolation region being formed in a side portion of the deep-trench region and the common-drain structure includes a common-drain diffusion region and different implant regions under the common-drain diffusion region for forming punch-through stops of the vertical transistor and the parasitic collar-oxide transistor. The deep-trench region comprises a lower capacitor node made of an n+ diffusion region being formed in a lower portion of a deep trench, a capacitor-dielectric layer being formed over the lower capacitor node, an upper capacitor node being formed over the capacitor-dielectric layer, a collar-oxide layer together with a capacitor-node connector being formed over the capacitor-dielectric layer and the upper capacitor node, a source conductive node being formed over the collar-oxide layer and the capacitor-node connector to act as a dopant diffusion source for forming a common-source diffusion region, an isolation silicon-dioxide node being formed over the source conductive node, and a conductive-gate node of a vertical transistor being formed on the isolation silicon-dioxide node. The second-type shallow-trench-isolation region being formed in a side portion of the deep-trench region comprises a second-type second or third raised field-oxide layer with a bottom surface level approximately equal to that of the collar-oxide layer and an n+ diffusion region being formed under the second-type second or third raised field-oxide layer. The vertical transistor comprises a capping conductive-gate layer being integrated with the conductive-gate node, a gate-dielectric layer being formed over a sidewall of the deep trench, a common-drain diffusion region of a second conductivity type being formed over an upper portion of the semiconductor substrate, and the common-source diffusion region of the second conductivity type being formed near the source conductive node for forming a first-type vertical transistor DRAM cell; and comprises the conductive-gate node being formed in an upper portion of the deep trench, a planarized common-gate conductive island integrated with a metal word-line being connected with a portion of the conductive-gate node, a gate-dielectric layer being formed over a sidewall of the deep trench, a common-drain diffusion region of a second conductivity type being formed over an upper portion of the semiconductor substrate, and a common-source diffusion region of the second conductivity type being formed near the source conductive node for forming a second-type vertical transistor DRAM cell. The common-drain region comprises the common-drain diffusion region having a shallow heavily-doped diffusion region formed within a lightly-doped diffusion region, different implant regions under the common-drain diffusion regions for forming punch-through stops and adjusting threshold voltages of the vertical transistor and the parasitic collar-oxide transistor, and a planarized common-drain conductive island integrated with a metal bit-line being formed on the common-drain diffusion region outside of a third sidewall dielectric spacer formed over a sidewall of the deep-trench region for forming the first-type vertical transistor DRAM cell; and comprises a common-drain diffusion region having a shallow heavily-doped diffusion region formed within a lightly-doped diffusion region, different implant regions under the common-drain diffusion regions for forming punch-through stops and adjusting threshold voltages of the vertical transistor and the parasitic collar-oxide transistor, and a common-drain conductive bit-line being formed at least over the common-drain diffusion region outside of a third sidewall dielectric spacer being formed over a sidewall of the deep-trench region for forming the second-type vertical transistor DRAM cell. The cell size of the vertical transistor DRAM structure can be fabricated to be equal to 4F2.
The vertical transistor DRAM structure of the present invention is used to implement two contactless vertical transistor DRAM arrays. A first-type contactless vertical transistor DRAM array comprises a plurality of first-type vertical transistor DRAM cells, a plurality of metal bit-lines integrated with planarized common-drain conductive islands being patterned to be aligned above a plurality of active regions, and a plurality of capping conductive-gate layers integrated with the conductive-gate nodes to act as a plurality of conductive word-lines being formed transversely to the plurality of metal bit-lines. A second-type contactless vertical transistor DRAM array comprises a plurality of second-type vertical transistor DRAM cells, a plurality of metal word-lines integrated with planarized common-gate conductive islands being patterned to be aligned above a plurality of active regions, and a plurality of highly conductive common-drain bus-lines acted as a plurality of conductive bit-lines being formed transversely to the plurality of metal word-lines.